Conventional fiber-reinforced composite materials have a limited application range due to low interfacial shear stress in spite of excellent mechanical properties.
A variety of grafting methods are developed to improve interfacial shear stress of fiber-reinforced composite materials. However, most of the methods disadvantageously require high-temperature thermal treatment processes, have considerably long manufacturing time and poor bonding strength between carbon fibers and metal oxide, and are inapplicable to commercialization.
In an attempt to improve interfacial shear stress between fibers and a matrix in fiber-reinforced composite materials, methods for reducing surface free energy by applying a variety of surface treatment methods to fiber surfaces and imparting functional groups thereto are actively researched. However, most methods cause deterioration in physical properties of fibers and optimization of treatment conditions is difficult.
Accordingly, grafting methods which are capable of improving interfacial shear stress of fiber-reinforced composite materials and are applicable to commercialization, while causing deterioration in physical properties of fibers have been developed. Grafting methods have an effect of improving physical interfacial shear stress based on interlocking effects by growing a rod, wire or belt form of metal oxide in a direction vertical to a fiber length on fiber surfaces or other substrates such as metal, polymer and ceramic substrates and the like.
Grafting methods include a variety of methods such as hydrothermal synthesis, carbothermal reduction, chemical vapor deposition and thermal evaporation. Most methods include forming metal oxide by using a solution in which metal cations are dissolved or performing thermal treatment using metal particles as a precursor. However, most methods disadvantageously require vacuum conditions or a high temperature of 500° C. or higher, entail thermal treatment or have a very long manufacturing time, have bad bonding strength between carbon fibers and metal oxide, and are inapplicable to commercialization through continuous processes. In addition, the methods cause deterioration in physical properties of fibers, have limited application fields and are inapplicable to commercialization due to high-temperature application.
A hydrothermal method, which is one of grafting methods, can form a rod, wire or belt form of metal oxide on a substrate surface at a low temperature of 100° C. or less. In general, a hydrothermal method is divided into two steps. The first step is to form a seed on a substrate surface by thermal treatment in a seed solution and the second step is to deposit and then grow ions on the seed. However, the hydrothermal method requires a long time of 4 hours or longer, has low commerciality and is difficult to apply to continuous processes.
Accordingly, there is an urgent demand for development of new methods for forming metal oxides that are simple and are applicable to continuous processes in consideration of commercialization and have low cost and high production efficiency.